zanderman, I agree about the Baker's cyst although Baker's cysts are often a consequence of early arthritis or other inflammatory processes. I would also suspect that bony lump could be an osteophyte which is also a manifestation of arthritis!
I suggest you ask your GP to refer you to a knee surgeon who will xray it and MRI it as well. It isn't unknown for 20 yr olds to get arthritis, especially with history of the trauma. sports and athletics. Arthritis is, after all, a form of micro-trauma which is why it's often referred to as "wear and tear" and it's no respecter of age!
Sorry to depress you but I do I suggest you go see a good knee surgeon and get it properly investigated. Depends where you are in Scotland but I can recommend a great chap in Sunderland!